The present invention relates to developer compositions for photographic elements comprising photosensitive coatings on substrates, or more particularly to developer compositions for removing the non-image areas of exposed, usually positive working color proofing films. In the art of lithographic printing, it is desirable to produce a color proof to assist a printer in correcting a set of photomasks which will be used in exposing printing plates. The proof should reproduce the color quality that will be obtained during the printing process. The proof must be a consistent duplicate of the desired halftone or line image, and should neither gain nor lose color. Visual examination of a color proof should reveal any defects on the photomask, the best color rendition to be expected from press printing of the material; the correct gradation of all colors; and whether there is a need for changing any of the colors and/or for altering the film photomask before making printing plates.
Color proofing films are generally composed of a transparent substrate, which may or may not have been treated for release, slip or adhesive properties. The transparent substrate is then be applied with a photosensitive coating comprising a photosensitizer, binding resins, colorants, acid stabilizers, surfactants and other art recognized components. Common photosensitizers include o-quinone diazides, diazonium condensates salts and photopolymerizable compositions.
There are several types of photographic color proofing films known in the art, namely, the surprint, overlay, and transfer types. In the overlay type of color proofing, an independent transparent plastic support is used for producing an image of each color separation film. A number of such supports carrying colored images are then superimposed upon each other and placed on a white sheet to produce a color proof. The overlay type of color proof has the disadvantage that the superimposed plastic supports tend to darken the color proofing sheet, and, as a result, the impression of the color proofing sheet thus prepared becomes vastly different from copies actually obtained by a conventional printing press proof. Its primary advantage is that it is quick and can serve as a progressive proof by combining any two or more colors in register.
In the surprint type of color proofing method, a color proofing sheet is prepared by successively producing images of different colors from different color separation films onto a single receptor sheet. This is done by utilizing a single opaque support and by applying toners, photosensitive solutions or coatings of photosensitive materials of corresponding colors on the opaque support in succession. An example of this approach is described in U.S. Pat. No. 3,671,236. An advantage of the surprint type of color proof is that the color saturation is not influenced by superimposed plastic supports. This method more closely resembles the actual printing and eliminates the color distortion inherent in the overlay system.
U.S. Pat. Nos. 4,260,673 and 4,093,464 describe positive working one-piece proofing systems based on orthoquinone diazides. In U.S. Pat. No. 4,093,464 a colored image is transferred to a receiver sheet after exposure and development. U.S. Pat. No. 4,260,673 describes transfer of a solid color layer to a receiver sheet prior to exposure and development. U.S. Pat. No. 4,659,642 teaches a positive working color proofing system which has a transparent substrate, a colored photosensitive layer on the substrate, and a top adhesive layer.
It is known in the art to prepare developing compositions for lithographic elements. Developer compositions remove the non-image portion of the plate after it has been imagewise exposed. The developer should also prevent removed non-image particles of the photosensitive surface from being re-deposited back onto the film. In the past, various methods of developing lithographic elements have been employed. Such methods include the use of compositions containing organic solvents, surfactants, salts, acids and other art recognized materials in solution. Typical prior art developing compositions are described in U.S. Pat. Nos. 2,754,279; 4,381,340 and German OLS No. 2,216,419.
It is most desirable to have a developer composition which is capable of completing development in a short period of time. The proper selection of the developer solution is very important. It is most desirable to produce a developer which is an aqueous solution of salts and surfactants in the absence of organic solvents since organic solvents are expensive and ecologically disadvantaneous to dispose of after use. If the developer solution is a good solvent of the light sensitive composition itself, the solvent will satisfactorily distinguish between the exposed areas and the unexposed areas, and the image area coating is not dissolved, swollen or removed, otherwise such films will have no practical value. However, it has been found that during the development of color proofing films, particularly in positive working films, there is a tendency for the exposed non-image coating areas not to be completely removed. This results in an undesirable background stain which distorts the image. Nonimage areas especially those comprising resin binders such as acetals may not fully dissolve out. A balance of developer properties is necessary where adequate speed of development is attained, there is essentially no redeposit of particles, and background stain. It is also desired that the developer be useful in processing machinery where it can be used over again. As such it is important that the developing power not significantly change from one proofing sheet to the next, and certaining not change from the front of the sheet to the rear of the sheet.
It is an object of this invention to provide a developer for both machine and hand development of color proofing films having resin binders in their photosensitive coatings, which will enable the operator to prepare a film free from redeposit and background stain, and having good development speed. These and other objects will become apparent in the detailed description of the preferred embodiment.